gia handbook

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Magnification

6

Gem Identification


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Table of Contents

Subject Page

Loupes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Loupe Lighting Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Gemological Microscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Focusing the Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Microscope Lighting Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Darkfield Illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Brightfield Illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Reflected Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Diffused Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Polarized Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Examining a Stone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Identifying Clarity Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Surface Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Inclusions in Natural Gems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Inclusions in Synthetics and Imitations . . . . . . . . . . . . . . . . . . . . . . 35

Identifying Assembled Stones . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Key Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40


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The Gemological Institute of America

Carlsbad, CA 92008

2003 The Gemological Institute of AmericaAll rights reserved: Protected under the Berne Convention.No part of this work may be copied, reproduced, transferred, or

transmitted in any form or by any means whatsoever without theexpress written permission of GIA.

Printed in the United States.

Cover photos: All by Terri Weimer/GIA

Facing page: Magnification makes this included crystal visible. Its presence proves that the host sapphire is a natural gem.


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MAGNIFICATIONI know its a rubyI get the right RI readings, and the dichroscope

confirms its doubly refractive, said Mike. But when I look at it under

the microscope, I cant tell whether its natural or synthetic.

I know. Its getting tougher to separate treated natural rubies from syn-

thetic ones. But let me take a look, said Joe. Theres usually something

some mineral crystals, clouds, or needlesthat proves its a natural gem.

Joe examined the ruby carefully under the microscope. I can see a

couple of melted mineral crystals close to the girdle, and that whitishcloud is probably the remains of some growth zoning.

But what about that area under the tabledoesnt that look like flux

to you? asked Mike.

It looks more like the remnants of heat treatment, replied Joe.

When youve seen as many treated natural rubies as I have, youll rec-

ognize them quickly. Looking at a lot of them is the best way to become

familiar with their characteristics.

12003 GIA. All rights reserved.

Mike Havstad/GIA

Alan Jobbins

A whitish, hexagonal area is a feature

that identifies some heat-treated, naturalMong Hsu rubies.


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Like Mikes ruby, many gems carry signs of their nature in the form of

characteristics called inclusions, which are enclosed within a gem or reachits surface from the interior. Some types of inclusionslike mineral

crystalsare remnants of the rocks that natural gems grow in. Others

like curved color banding and platinum plateletsare found in synthetic

gems and hint at the processes used by manufacturers to grow them.

Blemishes can help with identification, too. Blemishes are characteris-

tics like scratches and abrasions on a polished gems surface. And they

can often indicate a gems hardness. For example, a fairly soft gem like

6G E M I D E N T I F I C A T I O N

2

John Koivula/GIA

These inclusions look similar, but one is a flux inclusion in a synthetic ruby (above)and the other is a borax inclusion in the partially healed fracture of a heat-treatednatural ruby (left).

Duncan Pay

Dietmar Schwarz

As gem crystals grow, they often trap mineral inclusions from their environment. Theincluded minerals help identify a gem as natural. Magnification at 50X reveals stalk-like green amphibole crystals in this natural emerald.

InclusionA characteristic

enclosed within a gemstoneor reaching its surface fromthe interior.

BlemishCharacteristic or irregu-larity confined to the surface of apolished gemstone.


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demantoid garnet (Mohs 6.5) often has abraded facet edges, while a

hard gem like corundum (Mohs 9) usually doesnt.

Magnification can help you determine if a gem is treated or if it contains

internal fractures, vulnerable cleavages, or other structural defects. Its also

an important tool for separating natural gems from synthetics. This is a

vital separation because theres such a large value difference between many

synthetic gems and their natural counterparts of equivalent quality.

For example, its easy to separate emerald from other green gems like

chrome tourmaline, chrome diopside, green sapphire, and peridot using

MAGNIFICATION

3

Alan Jobbins

Youll usually see abraded facet junctions on gems that dont rate very high on theMohs hardness scale. This demantoid garnet shows abrasions on its crown and

pavilion facet junctions, along with a prominent horsetail inclusion.

Both by Nicholas DelRe/GIA

This pendant (right) contains diamonds, natural rubies, and synthetic rubies.Magnification reveals gas bubbles in the stone at bottom right (above), indicatingits a synthetic ruby.

Magnification is a valuable tool

for detecting treatments and forseparating natural gems from their

synthetic counterparts.


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the refractometer because each gem has a very different refractive index

(RI). Its much more challenging to tell if an emerald is natural or syn-

thetic. Thats because the physical and optical properties of many natural

and synthetic stonesincluding emeraldoverlap.

Magnification can be a very powerful tool, and the more you practiceusing it, the more skilled youll become at recognizing the features that

help you make a final determination. But its also important to keep up

with the latest industry information by reading gemological business and

scientific journals.

Gemologists use two types of magnifiers: loupes and microscopes.

Loupes are small, easy-to-carry magnifiers that come in a variety of

forms. Microscopes are much more sophisticated and capable of far

greater magnification, but theyre much less portable.


4

Terri Weimer/GIA

A gemological microscope is more versatile and provides greater magnification thana loupe, but a loupe is much more portable. GIA Gem Instruments carries a varietyof loupes and microscopes.

Practical experience and up-to-date

knowledge are the keys to using magnifi-cation successfully in gem identification.

You can keep up to date with theconstantly changing gem worldby reading trade publications likeGems & Gemology.


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LOUPES

Whats the most popular loupe for gem identification?

How do you light a gem to examine its surface?

How do you light a gem to examine its interior?

A loupes portability and affordability make it a versatile tool for gem

identification. You learned how to use a loupe in Assignment 2. By

practicing the technique outlined there, you can master the loupe and

maximize its effectiveness in the gem identification process.

Some jewelers use an eye loupe that attaches to eyeglasses or fits in

an eye socket like a monocle, leaving both hands free to examine a

stone or to work on a jewelry piece. But today, most gem professionals

use hand loupes. A hand loupe has a cover that doubles as a handle.

MAGNIFICATION

5

Reporters Press Agency/eStock Photo

The hand loupes lens cover serves as a handle when you examine a stone. Thecover protects the lens when the loupe isnt in use.


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Working distancethe distance from the lens to the surface of the

objectalso affects a loupes usefulness. Its determined by the loupes

focal distance, which is the distance from the surface of the lens to apoint thats in sharp focus.

The higher the magnification, the shorter the focal distance and

working distance. A 10X loupe focuses when an object is one inch away.

A 20X loupe doubles the magnification, but cuts the focal distance in

half, which means it focuses when the object is half an inch away. This

also cuts the working distance in half, leaving less of a margin before

the stone or its characteristics are out of focus. At 30X, the working

distance is even smaller.

MAGNIFICATION

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Terri Weimer/GIA

An uncorrected loupe shows both spherical and chromatic aberration.

Focal distanceThe distancefrom the surface of a lens to apoint thats in sharp focus.

Peter Johnston/GIA

A fully corrected triplet loupe uses a three-part lens to correct both spherical andchromatic aberration.


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This makes loupes with magnifications higher than 10X more difficult

to use effectively. The higher the loupes magnification, the closer you

have to get to the gem, and the harder it is to focus on an individual

feature within the stone.

The shortened working distance at higher powers also leaves less

room for lighting and makes it more difficult to light a stone effec-

tively. Another disadvantage is that it creates a shallower depth of

field. Depth of field is the distance thats sharp and clear in front of

and behind the pointsuch as a small inclusionthat youre focusingon. With higher-power loupes, the depth of field is very small. To keep

an object in focus, you need to keep both the loupe and the stone as

still as possible.

Another consequence of higher magnification is that the area of the

gem that you can examinethe field of viewbecomes smaller.

Despite these limitations, the loupe can be an amazingly revealing

instrument.


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Peter Johnston/GIA

When you use magnification to examine a gem, you have to consider the workingdistance from the lens to the gem, the focal distance from the lens to the character-istic youre examining, and depth of field, which is the area in front of and behindthe object youre examining. When you switch to higher magnification, you shorten

the working distance, focal distance, and depth of field.

Depth of fieldThe distancethats clear and sharp in frontof and behind the point youfocus on.

depth

offield

focaldistance

workingdistance

VIEWER

DEPTH OF FIELD

LOW MAGNIFICATION HIGH MAGNIFICATION


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LOUPE LIGHTING TECHNIQUES

There are two basic types of lighting you can use to examine transparent

gem materials with a loupe. One lets you examine the gems surface

while the other lets you see internal inclusions more clearly.

The first is reflected lightingstrong, direct lighting reflected off a

gems surface. Under reflected light, features like surface-reaching

fractures, abrasions, and cavities stand out strongly against the gems

polished surface.

You can use almost any strong light source, such as a desk lamp, a

high-intensity lamp, or a fiber-optic light source like a FiberLite. Holdthe stone face-up with tweezers and position the light source and the

stone so the light reflects off the stones surface. Tilt the gem until each

facet in turn shows a bright, shiny, reflective surface. Examine the stone

face-up first and then keep turning the stone until youve examined it

from every side.

The second techniquedarkfield illuminationlets you see into a

gems interior. With this form of illumination, inclusions within the gem

stand out strongly against a dark background.

MAGNIFICATION

9

Eric Welch/GIA

Darkfield lighting lets you examine the interior of a transparent stone. You can createit by taping a sheet of black paper to the back edge of a lampshade.

Reflected lightingIllumination ofa gems surface by reflecting lightfrom it.

Darkfield illuminationLighting ofa gemstone from the side againsta black, non-reflective backround.

Reflected light is best for examining a

stones surface.

Darkfield illumination works best for

examining the interiors of transparent

stones.


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You can use a desktop lamp and black paper to create darkfield illu-

mination for a standard 10X loupe. Follow these steps:

1. Use tape strips to hang a sheet of black, non-reflective paper from the

back edge of the lampshade.

2. Turn the room lights off. Turn the lamp on, and direct the lightstraight down.

3. Hold the stone face-up with tweezers. Position the stone so the girdle

plane lines up approximately with the front edge of the lampshade.

4. Examine the stones interior against the black background, adjusting

the stones position to find the best view. Keep the stone in the light

and the loupe out of the light. This directs light in from the sides of

the stone without creating too many distracting reflections off the

crown.

If youre trying to distinguish diffusion treatment, a third type of

lightingdiffused lightingcan be helpful. You can create diffused

lighting by placing a sheet of translucent white material, such as a facial

tissue, between the light source and the stone.

Using a loupe, you can perform almost any basic magnification test.

But for more powerful magnification needs, the gemological microscope

is the instrument of choice.

GEMOLOGICAL MICROSCOPES

Why is a binocular microscope the best choice for gem

identification?

Whats the best way to focus a gem microscope?

What are the various types of lighting needed for

examining gems?

With its sophisticated optical system, sturdy construction, integrated

lighting, and greater working distance, the gemological microscope can

almost always help you identify treatments and make the vital separation

between natural and synthetic gemstones.

Most microscopes designed for grading or identifying gems are

binocular, which means they have two sets of lenses. The binocular

optical system has a great advantage over the monocular system, which

has only one lens set. The binocular system produces a three-dimensional

image with normal orientation. This makes manipulating the gem mucheasier. Most monocular systems produce images that are flat, upside

down, and reversed.

There are four basic parts to a typical binocular gemological micro-

scope. The housing at the top of the microscope that contains the optical

components is called thepod. You can move the pod up and down to

change the instruments focus. You do this by turning the focus-control

knob on the instruments arm.


10

Eric Welch/GIA

You can create diffused lighting bytaping a translucent white sheet to thefront of a lampshade. It can help youdetect the characteristic color zoning indiffusion-treated corundum and curvedcolor banding in flame-fusion syntheticcorundum.

PodThe housing for a micro-scopes optical system, also calledthe head.


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Directly below the pod, theres a working platform called the stage,

where you place the gem for examination. Beneath that, theres housing

for a light source called the light well. Finally, theres the base, which

supports the microscope and contains the electrical controls.

The pod contains a complex system of lenses and prisms. The eye-

piece lenses are called the oculars, and the lenses nearest the stone are

the objectives. Most microscopes have eyepieces that you can adjust for

individual comfort. Some are equipped with plastic or rubber eyecups

that help eliminate extraneous light, keep your eyes at the correct distance

from the oculars, and make the microscope more comfortable to use.

The eyecups are removable, and some gemologists, especially those

who wear glasses, prefer to work without them.

MAGNIFICATION

11

A binocular microscopelike this one from GIA Gem Instrumentsis a superior toolfor any gemological task that requires magnification.

StageA microscopes workingplatform.

Light wellHousing for a micro-scopes light source, located

below the stage.

BaseThe support platform thatcontains a microscopes electricalcontrols.

OcularsThe eyepiece lenses ona gemological microscope.

ObjectivesThe lenses nearestthe stone on a gemological micro-scope.

pod

stage

light well

base

oculars

objectives

GEMOLOGICAL MICROSCOPE

zoom adjustment

focus adjustment overhead light

rheostat forlight well

overhead light switch


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You can determine the power of a microscopes magnification by

multiplying the power of the oculars by the power of the objectives.

For example, 10X oculars and 2X objectives give 20X magnification.

The typical range is 10X to 30X or 45X, but some models go up to

70X or more.

With some gem microscopes, you change magnification powers by

switching oculars. With others, you turn the objectives to one of several

settings (1X, 2X, or 3X, for example). But the most versatile models let

you zoom smoothly from one power to another. The zoom adjustment is

usually on the side or top of the pod. It can be a single knob or a pair of

knobs, with one on either side of the pod. A calibrated dial displays the

magnification of the objectives.

Many models let you attach a doubler, which is a 2X lens that screws

on under the objectives, doubling the power. You can accomplish most

gem identification with 10X to 45X magnification. Very few identifica-

tions require more than 90X.While higher magnification decreases the depth and width of field in

a microscope, its depth and width of field are much larger than a loupes

to begin with, so there are fewer practical problems. As with a loupe,

higher magnification makes it more difficult to light the stone properly,

but the lighting systems built into many gemological microscopes provide

effective illumination at higher powers.

The microscopes stage has an opening that allows light to pass

through from the light well below. Most gem microscopes have an iris

diaphragm at the top of the stage that you can open or close to control

the amount of light that comes up from the light well.

The stage might also have a number of sockets where you can attacha stoneholder, which has spring-loaded jaws designed to hold a gem.

Because it attaches to the stage, it holds the gem firmly in place and leaves

both hands free to operate the microscope or to record what you see. If

you use tweezers rather than a stoneholder, rest them against the edge of

the light well on the microscopes stage to hold your gemstone steady.

Theres often an overhead fluorescent light sourcea removable

source of daylight-equivalent lightmounted at the front of the stage.

The light well consists of a frosted glass or plastic cylinder inside a

reflective metal bowl. The microscopes light source is positioned at the

base of the bowl, immediately below the cylinder. Above that, at the base

of the cylinder, theres a bafflea small metal flap that can be opened

or closed. When its open, light comes directly through the opening to

light the stone from below. When its closed, the light is forced to come

up from the sides of the light well rather than through the opening. As

youll see, this is essential for darkfield illumination.

The microscopes base contains most of its electronics. On the back

of the base is a small knob called a rheostat that turns the internal light

bulb on or off and also controls the lights intensity.


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Iris diaphragmA device in themicroscopes stage that you canopen or close to control theamount of light coming from thelight well.

StoneholderA device thatattaches to a microscopes stageto hold a gem steady.

BaffleA small metal plate thatyou can close to prevent themicroscopes light from shining

directly through the stone frombelow.

Terri Weimer/GIA

The stoneholder attaches to the side ofthe microscopes stage and holds thestone in place. The iris diaphragm at thetop of the light well consists of a seriesof metal plates that slide together orapart to control the amount of light com-ing up through the light well. You oper-ate it by moving the lever on the left. Atthe bottom of the light well, theres ametal flap called the baffle. When itsclosed, it prevents light from shiningdirectly through the stone. You operateit by turning the knob on the right.


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FOCUSING THE MICROSCOPE

Its important to focus your microscope before you begin each work

session. You should also refocus it periodically during long work sessions.

Follow these steps:

1. Turn the well light on. Make sure the baffle is closed and the iris

diaphragm is shut down to a small opening. At 10X magnification and

using both eyes, focus on the edges of the iris diaphragms opening,

which is near the center of the field of view. Most gem microscopes

have one ocular (usually the left) that you can focus without the

focus-control knob. Remove that ocular and look through the right

ocular, keeping both eyes open.

MAGNIFICATION

13

All by Eric Welch/GIA

To set up a microscopes focus, begin by turning the well light on (top left). Next, make sure the baffle is closed and the irisdiaphragm has only a small opening (bottom left). Then, using 10X magnification and both eyes, turn the focus-control knob tofocus on the edges of the small opening in the iris diaphragm (right).


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2. Turn the focus-control knob until the iris diaphragms opening is in

focus in your right eye. Release the focus-control knob. Dont touch it

again until after you focus the left ocular.

3. Replace the left ocular. Remove the right ocular and look through the

left ocular, keeping both eyes open. Bring the iris diaphragms opening

into focus by turning the ring at the bottom of the left ocular.


14


Continue by removing the left ocular and looking through theright ocular. Keep both eyes open. Turn the focus-controlknob until the edges of the opening in the iris diaphragm arein sharp focus.

Next, replace the left ocular and remove the right ocular. Withboth eyes open, look through the left ocular. Focus on theedges of the iris diaphragm opening by turning the ring at thebottom of the left ocular.


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4. Replace the right ocular. Adjust the distance between the oculars for

your comfort. Confirm the focus by looking at the opening in the iris

diaphragm through both oculars at the same time. You should see a

single image, and it should look sharp and three dimensional. If it

doesnt, repeat the focusing process.

MAGNIFICATION

15

Finally, look through both oculars at the same time to confirmsharp focus on the edges of the iris diaphragm opening.

Continue by replacing the right ocular and adjusting the dis-tance between the oculars to suit your eyes.


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MICROSCOPE LIGHTING TECHNIQUES

A microscope offers many more lighting options than a loupe, and dif-

ferent types of lighting work best for seeing different gem features. The

most useful techniques are darkfield, brightfield, diffused, reflected, and

polarized lighting.

An overhead light source makes it easier to see surface characteristics.

Some microscopes come with a small fluorescent light or let you attachone to the stage for this purpose. You can also use a desk lamp as you

would with a loupe. A fiber-optic system like the FiberLite is a versatile

supplementary light source thats vital for some separation processes.

Varying the lighting can have a dramatic effect on the visibility and

appearance of characteristics, and what you can determine by examining

them. Surface characteristics visible in reflected light are invisible in

darkfield light. And internal characteristics visible in darkfield light are

invisible in reflected light.


16

Eric Welch/GIA

A GIA Gem Instruments FiberLite, like other fiber-optic sources of condensed light,is especially good for horizontal, oblique, and pinpoint lighting. Fiber-optic light isvaluable for identifying treatments and for separating synthetic from natural gems.


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DARKFIELD ILLUMINATION

Most gem microscopes have the built-in ability to provide darkfield

illumination for examining inclusions. You just have to turn on the

microscopes internal light source and close the baffle in the light well

so no light can enter the stone from directly below. Light enters the

stone from the sides and a little behind, making inclusions stand out

brightly against a dark background.

The degree to which a characteristic stands out against the surrounding

gemstone is called its relief. For example, included crystals are minerals

trapped within a gem as it grows. The brassy, metallic surfaces of pyrite

included crystals stand out readily in pale emerald, so theyre described

as having high relief.

An included crystals relief depends on its RI and often its color,

especially compared to the color of the host gem. A cluster of moderately

sized, colorless calcite inclusions in a blue sapphire might be much harder

MAGNIFICATION

17

Peter Johnston/GIA

Most gemological microscopes are designed with a baffle to provide the option ofdarkfield lighting.

With the baffle closed, no lightenters the stone from below. Light fromthe sides makes inclusions stand outdramatically against a dark background.

ReliefContrast between aninclusion and its host gem.

Included crystalA mineral crystaltrapped within a gem as it grows.

DARKFIELD ILLUMINATION

microscope stage

baffle closedto create darkbackgroundfor stone

light source

stoneholder


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to see than a few black chromite crystals scattered around the interior of

a pale green peridot.

Most included crystals are relatively easy to see under darkfield

illumination. Other characteristics, like liquid inclusionspockets in

gems filled with fluids and sometimes other materialsmight require

different lighting techniques because they tend to blend into the host


18

Karl Schmetzer

Low-relief inclusions like this spinel crystal in ruby (above) are not as easy to see ashigh-relief inclusions like the black chromite crystal in peridot (left).

Alan Jobbins

Liquid inclusionPocket in a gemthats filled with fluids and, some-times, gas bubbles and crystals.

John Koivula/GIA

Low-relief inclusions like this one, which contains a liquid, a gas, and a tiny crystal,are common in some emeralds from Colombia.


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gems background if you use darkfield.

Horizontal lighting is a pinpoint darkfield technique, where you direct

a narrow beam of light toward the side of the stone. A fiber-optic light

works best for this type of lighting. You can aim the light straight at the

stone or from an oblique angle. When you look at the stone from above,

pinpoint crystals and gas bubbles stand out as bright objects.

MAGNIFICATION

19

Both by Eric Welch/GIA

By using a fiber-optic light source to illuminate a stone horizontally and examiningthe gem with the well light both on (left) and off (right), you can see inclusions thatmight otherwise go undetected.

Eric Welch/GIA

Oblique lighting places the illumination from a fiber-optic light at an angle betweenhorizontal and overhead.

John Koivula/GIA

Horizontal lighting reveals minute fluxparticles in a Kashan synthetic ruby.


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BRIGHTFIELD ILLUMINATION

Brightfield illuminationsometimes called transmitted lightresults

when you open the light wells baffle so the light is transmitted directly

through the stone to your eye. To keep from being dazzled by the bright

light, close the iris diaphragm so the opening is smaller than the stone.

This will create focused, pinpoint illumination. If necessary, adjust the

intensity of the light source with the rheostat.

Brightfield illumination makes inclusions look dark and featureless

against a bright background, so it works well for seeing low-relief

features like curved striae in flame-fusion synthetics. Curved striae

are structures that represent the layers of crystal growth around the


20


With the light wells baffle open, brightfield illumination

transmits light up through a transparent stone to your eye.

You create brightfield pinpoint illumination by keeping the

baffle open and closing the iris diaphragm until its openingis smaller than the stone.

John Koivula/GIA

Brightfield pinpoint illumination reveals gas bubbles and curved striae in a flame-fusion synthetic ruby.

Curved striaeCurved growthpatterns seen in flame-fusionsynthetics.


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cylindrical or rod-shaped boule, which is a typical product of the flame-

fusion process.

The brightfield technique works best if you close the iris diaphragmand restrict the light source to a small opening directly under the stone.

This lets you see structures like curved striae more clearly.

You can actually create an effect similar to brightfield by rocking and

tilting the gem under darkfield lighting to create alternating dark and

bright backgrounds. This can be helpful for detecting flash-effect colors

in fillersseen mostly in fracture-filled diamond and emeraldor

determining if an inclusion is liquid or solid, transparent or opaque.

MAGNIFICATION

21

Both by Terri Weimer/GIA

By rocking and tilting a stone in darkfield lighting, you can create a brightfield effect.

Alternating dark to light can help you detect flash-effect colors in fillers or see if aninclusion is liquid or solid, transparent or opaque.

Both by John Koivula/GIA

Rocking and tilting this emerald to alternate dark (above) and bright (right) back-grounds revealed an orangy yellow to blue flash effect in the filler.

Use brightfield illumination to detect low-

relief features like curved striae.


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REFLECTED LIGHTING

Darkfield, brightfield, and diffused lighting make many inclusions easier

to see, but reflected light works better for surface characteristics and

some types of inclusions. To examine a gems surface characteristics,

you need to position the light source, which is usually the microscopes

overhead light unit, so the light reflects off the gem. The light should

strike the gems surface at close to a 90 anglethis is called vertical

overhead illumination.

Thin, flat inclusionslike the thin films seen in many rubiesare

easiest to see when light reflects from their surfaces. But you cant use

vertical overhead illumination to see internal characteristics like these

because reflections from the gems surface block your view of the

stones interior. Instead, you need to use a light sourcesuch as a fiber-

optic lightto direct a narrow beam of light at the stone from an

oblique angle. Light entering the stone from that angle reflects from

internal fractures, cleavages, and fingerprints, and makes them much

easier to see.

DIFFUSED LIGHTING

For diffused lighting, open the baffle and the iris diaphragm and cover

the stage opening with a white, translucent material. You can use facial

tissue or even the white plastic diffuser from the microscopes overhead

light source. Diffused light can help you detect liquid inclusions in natural

gems. Its especially good for detecting curved color banding in flame-

fusion synthetics. And its excellent for detecting uneven color zoning in

diffusion-treated corundum, where surface-related color often stands out

against the white background.


22


You can create diffused light by open-ing the baffle and placing a tissue or apiece of translucent white plastic on thestage over the well (top). Diffused light-ing reveals uneven color zoning in thisdiffusion-treated sapphire (bottom).

Eric Welch/GIA

To create vertical overhead illumination with a stone like this coral cabochon, anglethe stone so light strikes at about a 90 angle to its surface (above). Reflected over-head light and magnification reveal the characteristic straight, irregular, fibrousstructure in this shell cameo (left).


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POLARIZED LIGHTING

You can create polarized light by opening the microscopes baffle and

placing one polarizing filter over the light well and another between the

stone and the objectives. You can hold the second filter or attach it to the

objectives. Your microscope then functions as a magnifying polariscope.

Use this type of lighting to distinguish included crystals from similar-

looking gas bubbles or gas-filled cavities. Crystals might show interference

colors and are often surrounded by halos caused by strain, while cavities

or gas bubbles wont have these features.

MAGNIFICATION

23

Eric Welch/GIA

You can create polarized light with a microscope by opening the baffle, placing onepolarizing filter over the light well, and holding another between the stone and theobjectives. Rotate the handheld polarizing filter to cross the filters.

John Koivula/GIA

By revealing interference colors,polarized light can help you distinguishbetween included crystals and gasbubbles or gas-filled cavities, whichshow no color.

Liquid inclusions, curved color banding,

and uneven color zoning show up best in

diffused lighting.


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EXAMINING A STONE

As you become more comfortable with the microscope, the steps involved

in examining a stone will become second nature to you. The first step is

always to thoroughly clean the stone youre about to examine. This is

very important: Its easy to mistake grease and dust on the gems surfacefor abrasions or even inclusions.

Its important to hold the stone steady. If you use a stoneholder, attach it

to the stage and position the stone over the light well. If you hold the stone

in tweezers, rest them gently on the side of the light well.

Examine the stone thoroughly. If the gem is transparent to translucent,

examine both its surface and its interior. As you examine the stone, record

what you see on the Gem Identification worksheet. If possible, turn off

the other lights in your area while youre working.

1. Set the magnification to 10X. Always start at this magnification

level.

2. Start with the well light turned off, and use the microscopes overhead

light to examine the gems surface. Position the light and hold the

stone so light reflects from its facets. Look at the top and the bottom,

then all the way around the sides.

3. If your stone is transparent to translucent, examine its interior next.

Turn off the overhead light and turn on the microscopes internal light

source. Make sure the baffle is closed and the iris diaphragm is com-

pletely open.


24


To examine the surface of a stone, turn the well light off and use the microscopes overhead light. Rotate the stone to examineevery side.

Always start the examination process at

10X magnification.


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4. To examine the interior, start by focusing on the surface, then hold

the stone still and move the focus down slowly until the back of the

gem comes into focus. Slowly raise the focus back to the gems

upper surface.

5. Repeat the process from the top, from the bottom, and from every side

to make sure you view the interior of the stone from every possible

viewing angle.

6. Switch to higher-power magnification to identify any characteristics you

cant see at 10X. This will also help you determine the nature of hard-

to-see characteristics. If you have a microscope with a zoom system,

youll soon learn to move from low to high magnification with ease.

MAGNIFICATION

25


To examine a stones interior, turn the well light on, close the baffle, and turn theoverhead light off. Vary the focus between the stones upper and lower surfacesto thoroughly examine its interior. Turn the stone to several positions and repeatthe process.


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26

The splitting of light in a doubly refractive (DR) gem produces

doubling: doubled images of facet junctions, inclusions, and other

characteristics. To see doubling, you must look at the object through

the stonebut not in an optic axis direction.

Although birefringence, which is the cause of doubling, is a very

constant property, the amount of doubling you see varies with the

stones size. The larger the stone, the more doubling youll see. It

also depends on your viewing angle when you observe the stone.

Doubling is hard to see in gems like quartz and corundum, but

readily visible in calcite, synthetic moissanite, and synthetic rutile.

Its especially useful for proving that over-the-limits stones like

zircon are DR. With practice, you can even estimate birefringence

by judging the separation between the doubled images.

When you look for doubling:

Always use the same power, such as 10X or 20X.

Look through the stone to the opposite side. Look for doubledimages of facet junctions, inclusions, and scratches. Make sure

that an image isnt just a reflection. (This can occur close to facet

junctions.)

Look in at least three different directions to make sure youre notlooking down an optic axis. The strength of the doubling also

varies with direction. Estimate birefringence in the direction of

greatest doubling.

To confirm doubling with a microscope, hold a polarizing filterbetween the stone and the microscopes objectives. When you

rotate the filter back and forth about 90, the doubling appears and

disappears.

Seeing Doubling

All by Terri Weimer/GIA

If you look through a DR stone in an optic axis direction, you wont see doubling(left). As you rotate the stone farther away from the optic axis, the doubling keeps

increasing (center and right).


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MAGNIFICATION

27

Both by Terri Weimer/GIA

You might see doubling in some transparent DR stones when you examine themin darkfield lighting (left). You can confirm doubling by holding a polarizer betweenthe stone and the objectives and rotating it, making the doubling appear anddisappear (right).

Taijin Lu/GIA

When you look at a synthetic moissanite under magnification, you see doubleimages of its facet junctions (20X).


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Low power is best for detecting overall patterns of clarity character-

istics, while high power is best for identifying and examining individual

characteristics in detail. Use only as much magnification as you need to

identify an inclusion. If you magnify the feature too much, you might

miss important patterns that would be more obvious at lower power.

Now that youve been through the basic steps of examining gems

with both the loupe and the microscope, its time to identify the basic

characteristics youre looking for in natural and synthetic gemstones as

well as imitations.

IDENTIFYING CLARITY CHARACTERISTICS

Which inclusions are typical of natural gems?

Which characteristics are typical of synthetic and

imitation gems?

How can the microscope help you identify assembled

stones?

Clarity characteristics are often the key to a gems identity. They can

help separate a gem that grew naturally in the earth from one grownsynthetically in a laboratory. In rare cases, inclusions can even help

establish the geographic origin of an important natural ruby, sapphire,

or emerald. They can also usually tell you if a gem has been treated.

Later, in the separation assignments, youll read much more about the

inclusions that identify specific gems. This section is a brief overview to

remind you of the various types of characteristics youll encounter,

starting at the gems surface.


28

Both by Nicholas DelRe/GIA

Low magnification is best for detecting overall patterns of clarity characteristics(above), while high magnification is best for close examination of individualcharacteristics (left).


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SURFACE CHARACTERISTICS

As youve learned, you use overhead, reflected light to examine a stones

surface. Pay attention to areas that are vulnerable to damage, like the

girdle and culet.

You can hold large stones with tweezers or your fingers. Hold small

stones with tweezers. With a loupe, look for blemishes like scratches

and abrasions that indicate a stones hardness. Polishing wheels tend to

MAGNIFICATION

29

Alan Jobbins

Gems that grow in the earth often contain clarity characteristicssuch as mineralcrystals and needlesthat you wont see in synthetic stones. This helps you separatenatural gems from synthetics.

John Koivula/GIA

A gold or platinum platelet is a telltale sign of a hydrothermal or flux synthetic gem.The environments these synthetics grow in often contain those metals, and micro-scopic remnants end up in the stones.


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round the facet edges of gems with hardness below Mohs 7. If a gems

hardness is below Mohs 8 and it has been worn for any length of time,

its facet edges will probably be abraded.

Heat treatment leaves gems like colorless or blue zircon and tanzanite

slightly brittle, so these gems often have abraded facet junctions. Heat-

treated corundum often has melted surfaces along the girdle and in

other areas.

Glass and plastic imitations are often shaped in molds, resulting in

rounded facet junctions. As molten glass cools, it shrinks slightly, often

producing concave facets with surface texture that gemologists call the

orange-peel effect.

Fractures that reach the surface can sometimes help you identify a gem.

The surfaces of fractures in aggregates often have a granular, rough, or


30

Maha Tannous/GIA

Characteristics like melted surfaces are evidence of heat treatment in corundum.

Terri Weimer/GIA

Rounded facet junctions and the orange-peel effect are signs of molded glass.

The amount of wear a gemstone shows

can indicate its hardness.


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irregular texture, like the surface of a sugar cube. Fibrous materials like

tigers-eye quartz can have splintery fracture surfaces.

Most transparent gems, such as quartz, beryl, corundum, and tour-

maline, have a conchoidal, or shell-like, fracture surface. If the stone

has cleavage, you might see either flat cleavage faces or small con-

choidal fractures alternating with flat cleavages, creating a step-like

appearance.

Surface-reaching fractures can contain some of the oil or dye used

to conceal the fractures or to enhance the gems color. You can locate

them by examining the gems surface in reflected light. Once youve

found a surface-reaching fracture, switch to darkfield illumination to

follow the fracture as it extends into the gem and look for signs of a

filling material.

MAGNIFICATION

31

John Koivula/GIA

Reflected light is best for finding surface-reaching fractures like this one. Its beenfilled, which makes its length difficult todetermine. Switching to darkfield wouldhelp you detect the filling.

There are three methods you can use to distinguish between a trans-

parent gems external and internal characteristics. With a microscope,

its best to use fairly high magnification30X to 50X.

REFLECTED LIGHT

Reflected light is the best and most widely used method. Hold the

stone so light reflects from the surface the object appears to be on. If

the object is external, such as a piece of dust on a facet, it will stand

out. If its internal, youll see an unbroken, mirror-like reflection from

the facet.

PLANE OF FOCUS

Hold the stone so youre looking at the surface the object appears to

be on. Try to bring both the surface and the object into the sharpest

possible focus. If both come into focus at the same time, the object is

on the surface or extremely close to it. If the surface comes into focus

first, the object is inside the gem.

ROCKING

Choose a nearby feature you know is externalsuch as a facet

junction or a scratchto use as a reference. Then rock the stone

slowly back and forth while you watch the object in question and the

reference feature. If the object is on the same surface as the reference,

it will move the same amount. If its within the gem, the object will

move less.

Is the Characteristic Internal or External?

Peter Johnston/GIA

When you use the plane-of-focus tech-

nique, you know an object is on the facetsurface when the object and the surfaceare in focus at the same time (top). If theobject is below the surface, its out offocus when the facet surface is in focus(center), and in focus when the facet sur-face is out of focus (bottom).


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INCLUSIONS IN NATURAL GEMS

Natural gems often have an abundance of clarity characteristics. In

general, natural gems contain a far greater range of characteristics than

synthetic ones.

Natural gems grow in an environmentthe earths rockswhere

theyre in competition with many other minerals for the ingredients of

crystal growth. As they grow, they often trap other minerals as included

crystals. By comparison, synthetic and imitation stones grow in much

more controlled surroundingsthe pristine environment of the laboratoryor factory. As a result, theres less opportunity for them to acquire foreign

materials as they grow.

This means that when you see a range of different mineral inclusions,

you know youre looking at a natural gem. A natural ruby, for example,

might contain a variety of included mineral crystals. You might see

colorless, rounded calcite, zircon, or apatite crystals and dense patterns

of tiny, slender rutile needles that intersect to form silk.

Needles can be solid or hollow. If theyre hollow, they might be filled

with liquid or gas. Solid needles occur in corundum, garnet, and some

emeralds. Hollow needles are frequent features of chrysoberyl. In tour-

maline and beryl, the hollow needles are called growth tubes. Theyreoften much coarser than hollow needles in other gems, and might be

capped by tiny included crystals.

Gems that grow in mineral-rich, watery solutions often contain liquid

inclusions. Topaz, beryl, and quartz can have abundant liquid inclusions.

Sometimes an inclusion also contains a gas or a solid, or both. When

only two of those things are presenta liquid and, typically, a gasits a

two-phase inclusion. If all three are present, its a three-phase inclusion.


32

NeedleA long, thin inclusion thatcan be a solid crystal or a hollowtube that might be filled with liquidor gas.

SilkGroup of fine, needle-like

inclusions.

Two-phase inclusionA cavity ina gem filled with a liquid and,typically, a gas.

Three-phase inclusionA cavityin a gem filled with a liquid, a gas,and one or more crystals.

Edward Gbelin/GIA

Needle-like mineral inclusions in emerald (above) and ruby (left) mean these gems

are natural. The group of intersecting needles in the ruby is called silk.

Kari A. Kinnunen

Blocked crystal growth causes hollowor filled growth tubes in beryl.


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Sometimes, a gem might contain angular spaces that adopted the

shape and symmetry of the enclosing gem crystal when it cooled. They

look like mineral inclusions, but theyre not. These hollow areas arecalled negative crystals, and they usually contain a liquid or a gas, or

both.

Negative crystals are common in corundum, quartz, topaz, and beryl.

They can also occur in synthetic gems, so youll have to look for other

evidence to be sure the gem is natural. If you suspect youre looking at

a negative crystal, you can use polarized light to confirm it. Unlike a

solid mineral crystal, a negative crystal shows no strain colors.

MAGNIFICATION

33

Negative crystalAn angular,hollow space within a gem thatresembles a mineral inclusion.

John Koivula/GIA

Three-phase inclusions are evidence of naturally formed emeralds. They contain a

liquid, a solid, and a gas.

Eduard Gbelin/GIA

Negative crystals are angular, hollow spaces that usually contain a liquid and a gas.

Natural gems typically contain a far

greater variety of inclusions thansynthetic gems.


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Crystals often fracture during growth. Sometimes fluids seep into the

breaks and become trapped as the fracture recrystallizes. If a break

doesnt heal completely, it creates a pattern of small, disconnected fluidinclusions within the stone. Because of its appearance, the inclusion is

called afingerprint. Fingerprints can also consist of included crystals,

two-phase or three-phase inclusions, or negative crystals, as long as

they form a fingerprint-like pattern.

Inclusions can be so tiny and numerous that its hard to see them

individually, even at the highest magnification. When theyre numerous

enough, describe them as a cloud. A cloud is any hazy or milky area

that cant be described as a feather, fingerprint, or group of included

crystals or needles. Many diamonds, rubies, and sapphires contain

clouds.

As crystals grow, their growth stages often show up as color zoning.Color zoning is a pattern of alternating light and dark areas or of

different colors. Its often seen in gems like corundum, quartz, and

tourmaline. Its caused by variations in trace elements during crystal

growth.

In natural gems, the bands are straight and angular, following the

gems crystal structure. Synthetics can have straight or angular color

zoning, which indicate flux and hydrothermal growth processes, or


34

Michael Waitzman/GIA

Fingerprint inclusions can contain a variety of materials. The fingerprint in the spinel(above) contains crystals and negative crystals. The fingerprint in the first blue sap-phire (top left) contains negative crystals and two-phase or liquid inclusions. Theother blue sapphire (bottom left) has a fingerprint thats composed of liquid withinpartially healed fractures.

Both by Mike Havstad/GIA

CloudAny hazy or milky areathat cannot be described as afeather, fingerprint, or group ofincluded crystals or needles.

FingerprintInclusions that form apattern that often resembles ahuman fingerprint.

Robert Kane/GIA

Distinctly bluish clouds often occur inVietnamese rubies.


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curved color zoning, which means they were produced by flame-fusion

or pulling processes.

INCLUSIONS IN SYNTHETICS AND IMITATIONS

The inclusions in synthetic gems often indicate the growth process

the manufacturer used to produce them. You might see gas bubbles in

synthetics produced by the flame-fusion or pulling processes. Theyre

especially likely in flame-fusion synthetics, where they can be spherical,

elongated, or distorted. Spherical gas bubbles might have dark centers

that make them look like doughnuts.

The only untreated natural materials that contain gas bubbles are

natural glasses like obsidian and moldavite and natural resins like amber.

Gas bubbles occur in these natural amorphous materials, but almostnever in natural crystalline materials, except as part of two-phase or

three-phase inclusions or in the junction planes of assembled stones.

They might also occur in glass or plastic fillers or where mineral inclu-

sions were melted by heat treatment. Some natural gems can contain

rounded crystals that resemble gas bubbles.

Synthetics grown by the flux process usually contain inclusions that

are remnants of the medium that the ingredients for crystal growth were

dissolved in. Although theyre often thick and coarse looking, resembling

MAGNIFICATION

35

Rolf Schwieger

Angular color zoning (above) and straight color zoning (right) often occur in bluesapphires. The zoning follows the gems crystal structure.

John Koivula/GIA

John Koivula/GIA

Gas bubbles are common in flame-fusion synthetics like this manmade ruby.

The only untreated natural gem materials

that contain gas bubbles are natural

glasses and resins.


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icicles, they can also be delicate in appearance. Gemologists often

describe the delicate ones as wispy veils. They can resemble the

fingerprints in natural gems, but are often folded and twisted, while

fingerprints usually look flattened. Flux has higher relief than finger-

prints, and it might contain trapped gas bubbles. Its normally white,

but it can be colorless, yellow, orange, or brown.

As you read earlier, flame-fusion synthetics often show curved growth.Flame-fusion synthetic blue sapphires might show curved color banding.

Unlike curved striae, the bands are different tones of the same color

often with alternating colorless bandsbut theyre still curved. Youll

never see curved striae and curved color banding in natural crystals.

Flux and hydrothermal synthetics can show straight or angular color

zoning like that seen in natural material. The difference is that the zoning is

generally more uniform in the synthetics than it is in natural stones. This is


36

John Koivula/GIA

The thick, coarse, grainy texture of this inclusion is typical ofmaterial trapped in synthetic ruby crystals as they grow by theflux process.

Duncan Pay

The appearance of included flux can vary according to themanufacturers process. These yellow-to-orange flux inclusionsappear in a Ramaura synthetic ruby.

Karl Schmetzer

Some flux inclusions look delicate and are called wispy veils.These appear in a Russian synthetic alexandrite.

John Koivula/GIA

These large, white flux inclusions in a synthetic emerald areinterconnected with smaller flux channels.

John Koivula/GIA

Flame-fusion synthetic blue sapphireslike this often show curved color band-ing. Here, its in the gems crown.


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because of the more controlled environment the synthetics grow in.

Some hydrothermal synthetic emeralds contain nailhead spicules, which

are cone-shaped spaces extending from synthetic crystal inclusions.

Modern heat-treatment techniques can alter many natural corundum

inclusions so much that it can be difficult to separate them from some

flux-grown synthetics. Youll learn much more about the inclusions in

synthetic stones in Assignment 11.

IDENTIFYING ASSEMBLED STONES

Youve learned that assembled stones are composed of two or more pieces

of material glued or fused together to form one piece. In Assignment 2,

you learned some ways to identify them. Typically, a 10X loupe is useful

for detecting signs of assembly. Its important to be careful, however, not

MAGNIFICATION

37

Some hydrothermal synthetic emeralds can show liquid and two-phase inclusionsand fingerprint-like patterns.

John Koivula/GIA

In some hydrothermal synthetic emeralds, growth blockage can cause nailheadspicules to form. Theyre usually near the seed plate and point away from it in thegrowth direction.

Flame-fusion synthetics might show

curved striae or curved color banding, but

natural gems never do.


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to confuse the multiple girdle planes that can occur in natural gems with

the separation planes that indicate the presence of assembled parts.

The ability to examine a gemstone under magnification is an absolutely

indispensable skill for the gemologist. Now that you understand the

basics, you should get as much practice as possible. In later assignments,

as you test stones in your practice sets, take the time to look at each ofthem carefully. The gems in your practice sets can teach you a lot. In

the future, something you see in these sets might help you identify a

difficult gem.

Even if you have access to a microscope, dont rely on it exclusively.

Practice with a loupe as well, because on buying trips, youll usually

have to depend on it. In the next assignment, youll learn about the

spectroscope. Its another compact, portable instrument that you might

want to take along with you.


38

When inclusions that show in only natural or synthetic stonesappear together in the same stone, you know its assembled.This sapphire-synthetic sapphire doublet shows straight

growth and gas bubbles.

Rene Moore/GIA

Heat treatment damaged this sapphires surface, forcing thecutter to repolish the gem. Repolishing added extra girdleplanes. Dont mistake them for the separation planes that indi-

cate an assembled stone.

Curved color banding and a separation plane between the green crown and bluepavilion prove this is an assembled stone.


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MAGNIFICATION

39

Magnification is a valuable tool for detecting treatments and

for separating natural gems from their synthetic counterparts.

Practical experience and up-to-date knowledge are the keys

to using magnification successfully in gem identification.

A fully corrected 10X triplet loupe is an essential gemological

tool.

Reflected light is best for examining a stones surface.

Darkfield illumination works best for examining the interiors

of transparent stones.

Use brightfield illumination to detect low-relief features like

curved striae.

Liquid inclusions, curved color banding, and uneven color

zoning show up best in diffused lighting.

Always start the examination process at 10X magnification.

The amount of wear a gemstone shows can indicate its

hardness.

Natural gems typically contain a far greater variety of

inclusions than synthetic gems.

The only untreated natural gem materials that contain gas

bubbles are natural glasses and resins.

Flame-fusion synthetics might show curved striae or curved

color banding, but natural gems never do.


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40

BaffleA small metal plate that you can close to

prevent the microscopes light from shining directlythrough the stone from below.

BaseThe support platform that contains a micro-scopes electrical controls.

BlemishCharacteristic or irregularity confined tothe surface of a polished gemstone.

Chromatic aberrationColor distortion caused bythe inability of a lens to bring the various coloredwavelengths of light into focus at the same distance.

CloudAny hazy or milky area that cannot bedescribed as a feather, fingerprint, or group ofincluded crystals or needles.

Curved striaeCurved growth patterns seen inflame-fusion synthetics.

Darkfield illuminationLighting of a gemstone fromthe side against a black, non-reflective background.

Depth of fieldThe distance thats clear and sharpin front of and behind the point you focus on.

FingerprintInclusions that form a pattern that oftenresembles a human fingerprint.

Focal distanceThe distance from the surface of alens to a point thats in sharp focus.

Fully corrected triplet loupeA loupe that containsa three-part lens that magnifies and corrects forspherical and chromatic aberration.

Included crystalA mineral crystal trapped within agem as it grows.

InclusionA characteristic enclosed within a gem-stone or reaching its surface from the interior.

Iris diaphragmA device in the microscopes stagethat you can open or close to control the amount oflight coming from the light well.

Light wellHousing for a microscopes light source,

located below the stage.

Liquid inclusionPocket in a gem thats filled withfluids and, sometimes, gas bubbles and crystals.

NeedleA long, thin inclusion that can be a solidcrystal or a hollow tube that might be filled withliquid or gas.

Negative crystalAn angular, hollow space withina gem that resembles a mineral inclusion.

ObjectivesThe lenses nearest the stone on a

gemological microscope.

OcularsThe eyepiece lenses on a gemologicalmicroscope.

PodThe housing for a microscopes optical system,also called the head.

Reflected lightingIllumination of a gems surfaceby reflecting light from it.

ReliefContrast between an inclusion and its hostgem.

SilkGroup of fine, needle-like inclusions.

Spherical aberrationBlurring around the edgesthat occurs when a lens cant get an entire imagein focus at the same time.

StageA microscopes working platform.

StoneholderA device that attaches to a micro-scopes stage to hold a gem steady.

Three-phase inclusionA cavity in a gem filled witha liquid, a gas, and one or more crystals.

Two-phase inclusionA cavity in a gem filled witha liquid and, typically, a gas.

key terms


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MAGNIFICATION

41

ASSIGNMENT6

QUESTIONNAIRE

Each of the questions or incomplete statements below is followed by several possible answers. Choose

the ONE that BEST answers the question or completes the statement. Then place the letter (A, B, C, or

D) corresponding to your answer in the blank at the left of the question.

If youre unsure about any question, go back, review the assignment, and find the correct answer.

When youve answered all the questions, transfer your answers to the answer sheet.

________1. Diffused lighting is most effective for detecting

A. thin, flat inclusions.

B. high-relief inclusions.

C. surface features like glass-filled cavities.

D. curved color banding in flame-fusion synthetics.

________2. A fringe of color that occurs when a lens focuses different wavelengths of light at

different distances is a result of

A. astigmatism.

B. full correction.

C. spherical aberration.

D. chromatic aberration.

________3. The distance from the surface of a lens to a point thats in sharp focus is called

A. relief.

B. depth of field.

C. focal distance.

D. spherical aberration.

________4. If a microscopes oculars are 15X and the zoom adjustment is set at 2X, the

magnification is

A. 7X.

B. 15X.

C. 17X.

D. 30X.

IF YOU NEED HE LP: Contact your instructor through GIA online, or call 800-421-7250 toll-free in the US and Canada, or 760-603-4000; afterhours you can leave a message.

CONTINUED NEXT PAGE...


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42

________5. The amount of wear a gemstone shows can indicate its

A. density.

B. hardness.

C. optic character.

D. specific gravity.

________6. Under the microscope, if the inclusion and the facet surface are both in focus at the same

time, the inclusion is probably

A. a fracture.

B. high relief.

C. deep inside the stone.

D. very near the surface.

________7. A long, thin inclusion that can be a solid crystal or a hollow tube is called a

A. cloud.

B. needle.

C. fingerprint.

D. negative crystal.

________8. A natural, untreated gem material that might contain gas bubbles is

A. ruby.

B. peridot.

C. obsidian.

D. diamond.

________9. The contrast between an inclusion and its host gem is called

A. relief.

B. saturation.

C. fluorescence.

D. distinctiveness.

________10. The small metal flap that can be closed to prevent a microscopes light from shiningdirectly through the stone is called the

A. pod.

B. baffle.

C. ocular.

D. light port. CONTINUED NEXT PAGE...


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MAGNIFICATION

43

________11. To examine a gems surface, use

A. reflected lighting.

B. darkfield lighting.

C. monochromatic light.

D. brightfield illumination.

________12. If you find curved striae or curved color banding in a gem, you know it is

A. natural.

B. synthetic.

C. heat treated.

D. fracture filled.

________13. You can be sure that a gem is natural if it contains

A. a feather.

B. wispy veils.

C. straight color banding.

D. a range of different mineral inclusions.

________14. Which of these would probably have concave facets?

A. Molded gems

B. Enhanced gems

C. Assembled gemsD. Synthetic materials

________15. An angular, hollow space within a gem that resembles a mineral inclusion is called a

A. cloud.

B. cavity.

C. feather.

D. negative crystal.

CONTINUED NEXT PAGE...


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44

Distance Education students: The following questions ask you to examine the stones in the set youre

currently working with. Choose the best answer to each question and continue filling in your answer

sheet as you did with questions 1 through 15.

________16. Which of the following do you see in stone 4 using 10X magnification?

A. Doubling

B. Wispy veils

C. Gas bubbles

D. Curved striae


A. Wispy veils

B. Curved striaeC. Included crystals

D. Three-phase inclusions


A. Curved striae

B. Flux inclusions

C. Nailhead spicules

D. Natural inclusions


A. Wispy veils

B. Curved striae

C. Included crystals

D. Three-phase inclusions


A. Wispy veils

B. Negative crystals

C. Included crystals

D. Curved color banding


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1. Introduction

2. General Observation

3. Refraction and the Refractometer

4. Polariscope Testing

5. Pleochroism and the Dichroscope

6. Magnification

7. Selective Absorption and the

Spectroscope

8. Fluorescence and Phosphorescence

9. Additional Tests

10. Separation and Identification

11. Separating Natural Gems from

Synthetics and Imitations

12. Detecting Gem Treatments

13. Separating Red, Pink, and

Purple Gems14. Separating Blue and Violet

Gems

15. Separating Green Gems

16. Separating Orange, Yellow,

and Brown Gems

17. Separating Colorless, White,

Gray, and Black Gems

18. Identifying Rough Gems,

Parcels, and Mounted Gems

19. Advanced Laboratory Testing

gia handbook

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